First, and not directed at Christopher, I’m would be amazed if anyone on this board didn’t already realize that Max V, by definition, occurs when acceleration ceases.
I don’t know if this advances the discussion with respect to sprinting, but if I understand your question, while the earth’s gravity is a downward force, an object (it’s center of mass) can be moved in a horizontal direction by the force of gravity if it is touching the ground. A simple example is a walking stick balanced at vertical and then released. The force of gravity and the ground force will cause the stick to rotate as it falls, and as it rotates, the center of mass moves both vertically and horizontally. When the stick comes to rest, depending on friction and other second order affects, the center of mass may have moved horizontally on the order of L/2 where L is the length of the stick.
Christopher, you bring up an intersting point, one that is valid but also confusing. A sprinter experiences the same phenomenom with every step. Expecially in the earlier phases, he has a lot of forward lean and gravity assists by attempting to rotate his torso forward and to the ground. He compensates by applying tremendous horizontal force to keep the lower part of his body under the upper part. If he didn’t, he would fall flat on his face, just like the walking stick. Instead of a falling stick, think of a motorcycle riding a wheelie. The front end wants to fall, and will, unless enough horizontal force is applied to counter the rotational forces. GCT decreases as speed advances, the sprinter has less and less time to generate enough horizontal force to counter the rotational forces and is forced to gradually bring his torso into a nearly vertical posture. Many things are now working against him…short GCT limiting his abililty to generate forces of any kind, an upright posture which not only limits any rotational forces that could be redirected into the horizontal plane, but one that puts him in an disadvantaged postion to create his own horizontal forces, not to mention the ever increasing forces of resistance working against him.
While I realize some would like to skip right past the discussion of what the actual rate limiting factors are and start designing a training program, I think there is still a tremendous amount to learn about the physics of sprinting and determining what actually limits the athletes ability to accelerate beyond his Max V. While I don’t want to stir that hornets nest again, IMHO there are a number of obvious clues that suggest that it is not simply the athlete’s inability to generate enough vertical force.